For example, this type of hybrid system is disclosed in following Patent Literatures 1 to 3. In a hybrid system of Patent Literature 1, five gear positions and a reverse gear are arranged as a first transmission and five gear positions and a reverse gear having gear ratios different from those of the five gear positions and the reverse gear of the first transmission are arranged as a second transmission. The first and second transmissions include rotary shafts (input shafts) connected to different rotational elements of a differential device and a common output shaft. The hybrid system performs EV (electric vehicle) driving by controlling the first transmission to be in a neutral state and transmitting an output of a motor/generator to a driving wheel through the second transmission. The motor/generator during the EV driving outputs negative motor torque. When an engine is started from the EV driving, the motor/generator is allowed to output positive motor torque and the motor torque is transmitted not only to a driving wheel but also to the engine to increase an engine speed.
In a hybrid system of Patent Literature 2, odd-numbered gears and even-numbered gears of a dual clutch transmission are used as first and second transmissions, respectively, and a first input shaft of the odd-numbered gears and a second input shaft of the even-numbered gears are connected to different rotational elements of a differential device. Meanwhile, in the dual clutch transmission, a reverse gear is arranged on the second input shaft. In a hybrid system of Patent Literature 3, a so-called automatic control manual transmission is used as a first transmission and high and low two gear positions are arranged as a second transmission; an input shaft and a rotary shaft of the high and low two gear positions of the manual transmission are connected to different rotational elements of a differential device.